*{
	margin: 0;
	padding: 0;
}

.box{
	margin: 0 auto;
	width: 1000px; //必须要写，否则不居中
}

.backtotop{
	width: 40px;
	height:80px;
	position: fixed;
	bottom: 30px;
	background-color: gray;
	left: 50%;
	margin-left: 600px;
}
.backtotop:hover{
	background-color: gainsboro;
}
